November 13, 2006
People with high levels of a fatty acid known as DHA, a "good" fat found in fish, may have a lower risk of developing Alzheimer's disease. The findings, from the large and ongoing Framingham Heart Study, bolster a growing body of evidence that heart-healthy foods like fish are good for the brain. The study appeared in the Archives of Neurology, one of the journals of the American Medical Association.
DHA, or docosahexaenoic acid, is what's known as an omega-3 fatty acid, a heart-healthy polyunsaturated fat found in high amounts in most types of fish. It appears to be important for a well-tuned central nervous system, too. Fats and oils make up about 50 to 60 percent of the brain's dry weight, and DHA is the most abundant fatty acid found in the cell membranes of the brain's gray matter. Studies done in the past 20 years found that DHA is important for a variety of brain cell components and functions.
The current study found that people with high blood levels of DHA were least likely to develop Alzheimer's or other forms of dementia. Other so-called "good fats," such as EPA (eicosapentanoic acid), did not have a similar beneficial effect. People who eat fish regularly tend to have high DHA levels in the blood.
DHA joins a growing list of factors that appear to affect a person's chances of developing Alzheimer's disease years down the road. Advancing age, family history, and particular genes you inherit from your parents have all been found to determine the risk of Alzheimer's disease and other forms of dementia in seniors. Recent studies have found that high levels of homocysteine, an amino acid that is derived from proteins in the diet and that can accumulate in the blood and contribute to heart disease, increase the risk for Alzheimer's disease as well.
What the Study Showed
In the current study, researchers at Tufts University in Boston studied the link between DHA levels in the blood and dementia in 899 seniors (average age 76). All were part of the population-based Framingham Heart Study, which has followed a large group of men and women over many years. The participants provided blood samples and underwent regular psychological and memory testing for an average of nine years. A subgroup of 488 also filled out a questionnaire assessing their diet, including answers about how much fish they ate. None of the participants had Alzheimer's disease or other forms of dementia at the beginning of the study. During the nine-year study period, 99 out of 899 participants developed some form of dementia, including 71 with Alzheimer's disease.
After controlling for other known risk factors for dementia, including age and homocysteine levels, the researchers found that men and women having the highest DHA levels had a 47 percent lower risk of developing dementia and 39 percent lower risk of developing Alzheimer's disease than those with lower DHA levels. Among the participants who completed the dietary questionnaire, those in the top quarter of blood DHA levels reported that they ate an average of three fish servings a week (providing an average of about .18 grams of DHA a day). The men and women with lower DHA levels ate substantially less fish.
DHA levels in the blood vary by the degree to which the liver converts alpha-linolenic acid, an essential fatty acid, to DHA and also according to the amount of DHA in the diet. "In our study, the correlation between [blood] DHA content and fish intake was significant, indicating that fish intake is an important source of dietary DHA," the authors write. "In the future, it will also be important to determine whether combined dietary supplementation with DHA can decrease further mental deterioration in patients with established dementia."
In addition to fish, DHA can be produced in the body from foods high in alpha-linolenic acid. Such foods include various vegetables oils, soybeans, walnuts, and wheat germ. Babies also get DHA from human breast milk.
"The level of DHA in the brain has been shown to be very important for learning ability and memory in early life in studies of rodents, baboons, and humans," writes Dr. Martha Clare Morris of Rush University Medical Center in Chicago, in an editorial accompanying the article. "It is only recently that the omega-3 fatty acids have been investigated for their importance to the aging brain. The DHA composition of the brain decreases with age as a result of increased oxidative damage to the lipid membranes," that is, damage to cell membranes resulting from highly-reactive oxygen-rich chemicals made by the body.
Research indicates that consuming more DHA in the diet later in life increases DHA levels in the blood and may increase levels in the aging brain as well. However, more studies are needed to determine whether omega-3 supplements, including fish and krill oils high in DHA, could prevent dementia, she concludes.
One recent study from Sweden, reported last month, found that nutritional supplements high in omega-3s like DHA may provide benefits for the very earliest stages of Alzheimer's disease. The "good" fats did not help in the more advanced stages of disease, however. [See the article, "Omega-3s, the Fats in Fish Oils, May Benefit Very Mild Alzheimer's Disease."]
Earlier population studies also suggest that people who eat a lot of fish, especially oily deep water fish like salmon and mackerel, have a lower rate of Alzheimer's. Diets high in DHA also led to a reduction in beta-amyloid, the sticky protein that builds up in the brains of those with Alzheimer's, in mice.
More study is needed before doctors can routinely recommend fish oils or supplements rich in DHA to treat or prevent Alzheimer's disease. In the meantime, it may be a good idea to "eat your fish," just as mom advised.
For more on the latest treatments for Alzheimer's and keeping the mind sharp, visit www.ALZinfo.org, The Alzheimer's Information Site.
Ernst J. Schaefer; Vanina Bongard; Alexa S. Beiser; Stefania Lamon-Fava; Sander J. Robins; Rhoda Au; Katherine L. Tucker; David J. Kyle; Peter W. F. Wilson; Philip A. Wolf: "Plasma Phosphatidylcholine Docosahexaenoic Acid Content and Risk of Dementia and Alzheimer Disease: The Framingham Heart Study." Archives of Neurology, Volume 63, Number 11, November 2006; pages 1545-1550.
Martha Clare Morris: "Docosahexaenoic Acid and Alzheimer Disease," editorial. Archives of Neurology, Volume 63, Number 11, November 2006, pages1527-1528.
Press release, American Medical Asssociation.